The Queen will die. That may come as a surprise to some of this country's more devoted monarchists, but it's true. And when she does, according to the nonsensical and bizarre ragbag of habits and laws we call a constitution, her son Charles will become Britain's head of state. No vote, no discussion, no choice. The moment his mum is declared dead Charles is declared king.

In a week when the Queen turns 83 and Charles is yet again denounced for meddling in politics, this is a matter that is increasingly in need of proper scrutiny and a full-throated public debate.

In the context of modern society and British democracy there is no place for a monarch who speaks out on political issues. The whole raison d'etre of the monarchy is that the monarch remains silent, letting the politicians run things, and in return she gets to keep the money, palaces and job. Otherwise we would have a person in a position of considerable influence who cannot be controlled or chosen by the people. So if a future king wants to break with their side of the bargain, it's time we broke with ours.

Prince Charles has made it abundantly clear where he stands on numerous issues, from architecture, the environment, health and education. He speaks out without fear of being made accountable for his views and actions and he has made it clear he intends to continue in this vein once he's on the throne.

To those who argue it's good to have a non-partisan champion for these causes I would say this: Charles's number one priority is to promote himself and his interests, not good causes, and while being funded by the taxpayer he refuses any serious level of accountability or scrutiny.

On the environment he has routinely been exposed as, at best, a naive meddler, and at worst a hypocrite. Just the other day James Gray wrote on the Republic blog about how Charles' flagship charity, The Prince's Foundation for Integrated Health, invests heavily in oil, gas and arms. Charles has recently been busy promoting Rainforest Bonds as a way to save the planet, a measure supported by the World Bank and Silvio Berlusconi but opposed by many leading environmentalists. He talks the talk but would prefer to fly the walk – taking a chartered flight and over a dozen staff with him every time he visits another country to lecture on saving the planet.

On health, he lobbies the government to include "alternative" medicines within the remit of the NHS. On architecture he is now busy trying to bulldoze his way through the democratic planning process to ensure his views are adhered to.

If current polls are anything to go by this is going to be a problem for David Cameron rather than Gordon Brown. While his charity work suggests Charles may be a fellow Tory, the issues the heir takes on are bound to bring him into direct conflict with the government, no matter which party is in power. And that is the problem. That is why Charles has to choose between being a prince or a political activist.

Just imagine for a moment a situation arising where Charles is king and he is publicly advocating a particular policy on, say, health. He doesn't have to do or say much, mind you. Just let it be know what his views are and let his PR team do the rest. Then imagine that the king's position is opposed by the government. What we will have is a very serious constitutional crisis.

At best it will pit the unaccountable monarch against the elected government in a popularity contest weighted in the king's favour. The government can press ahead with its plans and will get its way, but at what political cost? At worst it will create a deadlock and perhaps even foster an environment in which the government feels obliged to actually listen to the views of the king when making policy.

A political King Charles could set British democracy back 200 years. This is a serious risk and a serious problem. There is only one intelligent and legitimate solution: set Charles free to speak his mind, and give Britain a genuinely democratic, republican constitution, one that will secure and guarantee our democratic rights for generations to come.